The present invention relates to implantable medical devices. In particular, the invention relates to a subcutaneous implantable cardioverter defibrillator (SubQ ICD) which receives a rhythm signal wirelessly from an implantable cardiac rhythm sensor positioned on or close to the epicardium.
Implantable cardioverter defibrillators are used to deliver high energy cardioversion or defibrillation shocks to a patient's heart when atrial or ventricular fibrillation is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met. Defibrillation shocks are typically delivered when fibrillation criteria are met, and the R-wave cannot be discerned from signals sensed by the ICD.
Currently, ICD's use endocardial or epicardial leads which extend from the ICD housing through the venous system to the heart. Electrodes positioned in or adjacent to the heart by the leads are used for pacing and sensing functions. Cardioversion and defibrillation shocks are generally applied between a coil electrode carried by one of the leads and the ICD housing, which acts as an active can electrode.
A SubQ ICD differs from the more commonly used ICD's in that the housing is typically smaller and is implanted subcutaneously. The SubQ ICD does not require leads to be placed in the bloodstream. Instead, the SubQ ICD makes use of one or more electrodes on the housing, together with a subcutaneous lead that carries a defibrillation coil electrode and a sensing electrode.
The absence of endocardial or epicardial electrodes make rhythm and arrhythmia sensing more challenging with the SubQ ICD. Sensing of atrial activation is limited since the atria represent a small muscle mass, and the atrial signals are not sufficiently detectable thoracically. Muscle movement, respiration, and other physiological signal sources and environmental noises also can affect the ability to sense ECG signals and detect arrhythmias with a SubQ ICD.